1. Field of the Invention
The present invention pertains to the field of thin films for use in integrated circuits, and particularly ferroelectric thin films. More specifically, a specialized capping layer enhances the performance of ferroelectric thin films.
2. Statement of the Problem
Ferroelectric materials are characterized by their ability to retain an induced polarization state even in the absence of an applied electric field. If the polarization state in one direction is identified as logic xe2x80x9c0xe2x80x9d polarization state and the polarization state in the opposite direction is identified as a logic xe2x80x9c1xe2x80x9d polarization state, and appropriate circuitry is provided to sense the polarization state, ferroelectric thin films can be used as the information storage medium of a high speed nonvolatile computer memory. It is known that such a ferroelectric memory can be made by substituting a ferroelectric material for the dielectric capacitor material of a conventional DRAM capacitor circuit and making appropriate changes in the read and write circuits to utilized the ferroelectric film as the information storage medium. See, for example, U.S. Pat. No. 5,784,310 issued Jul. 21, 1998 to Cuchiaro et al. This substitution converts the DRAM cell to a nonvolatile memory cell due to long-term retention of an induced polarization state in the ferroelectric material even in the absence of an applied field. It is also possible to make a ferroelectric memory cell consisting of a single field effect transistor due to the nonvolatile polarization state of ferroelectric thin films, as described in U.S. Pat. No. 5,780,886 to Yamanobe et al.
A problem arising in the use of thin film ferroelectrics is that point charge defects at the thin film surfaces have the effect of screening the applied field due to the presence of induced charge at the thin film surface creating a field opposite to the applied field. Thus, some of the interior ferroelectric domains of the crystal are never exposed to a field having sufficient magnitude to completely polarize the domains. The polarization performance of the films suffers as a result of this field screening. Ferroelectric memory densities are limited by the magnitude of residual polarization that may be obtained from the ferroelectric material. In addition, prior thin film ferroelectric materials typically have high polarization fatigue rates making the memories unreliable in long-term use because the magnitude of polarization decreases with use. Further, it is also possible for the polarization hysteresis curves of thin film ferroelectrics to shift or imprint relative to a zero voltage or zero field value. Ultimately, with either the fatigue or imprint problems, control circuitry that is coupled with known ferroelectric materials will be unable to read the fatigued polarization state of the materials and, therefore, unable to store or retrieve bits of information. Thus, there exists a need to increase the polarization of thin film ferroelectrics for the purpose of improving ferroelectric memories.
The fatigue and imprint problems can be largely overcome through the use of layered superlattice materials, as reported in U.S. Pat. No. 5,784,310 issued Jul. 21, 1998 to Cuchiaro et al. Ferroelectric perovskite-like layered superlattice materials are a known class of self-ordering crystals, and have been used in thin films suitable for use in integrated circuits, e.g., as reported in U.S. Pat. No. 5,519,234 issued May 21, 1996 to Araujo et al. The term xe2x80x9cperovskite-likexe2x80x9d usually refers to a number of interconnected oxygen octahedra. A primary cell is typically formed of an oxygen octahedral positioned within a cube that is defined by large A-site metals where the oxygen atoms occupy the planar face centers of the cube and a small B-site element occupies the center of the cube. In some instances, the oxygen octahedra may be preserved in the absence of A-site elements.
The layered superlattice materials are characterized by an ability to find thermodynamic stability in layered structures. Disordered solutions of superlattice-forming metals, when exposed to thermal treatments, spontaneously form a single layered superlattice material compound having intercollated layers of perovskite-like octahedrons and a superlattice generator such as bismuth oxide. The resultant self-ordered structure forms a superlattice by virtue of a dual periodicity corresponding to the repeated layers. The layered superlattice materials have this self-ordering ability and, consequently, are distinct from semiconductor heterolattices which require the deposition of each layer in a separate deposition step.
It is known that the polarizability of layered superlattice materials is reduced if stoichiometric precursors are used, since some elements, such as bismuth, are more volatile and are disproportionately removed from the materials during drying and annealing. Therefore, precursors using excess amounts of these volatile elements are often used so that, after drying and annealing, the resulting material is approximately stoichiometric. Bismuth gradients have also been used to obtain essentially stoichiometric final layered superlattice materials. See, for example, U.S. Pat. No. 5,439,845 issued Aug. 8, 1995 to Watanabe et al. While the devices using a gradient show enhanced polarizability, they also must be relatively thick because of the multiple layers, resulting in lower density of the ferroelectric memory.
There remains a need to obtain greater residual polarization values from thin film ferroelectrics and, particularly, the layered superlattice materials, for the purpose of increasing the density of ferroelectric memories and other integrated circuits that contain ferroelectrics.
The present invention advances the art and overcomes the aforementioned problems by providing improved thin film ferroelectric devices having an enhanced magnitude of residual polarization. These improvements derive from the use of a capping layer between the electrode and the ferroelectric material to compensate defects at the interface between the ferroelectric material and the electrode. Improvements in residual polarization measurements as large as 32% have been derived from the use of the invention.
A ferroelectric device according to the invention includes a substrate supporting a thin film ferroelectric layer selected from the group consisting of perovskites and self-ordering layered superlattice materials. The ferroelectric material is xe2x80x9ccappedxe2x80x9d on one or both the top and bottom side by a capping layer. Preferably, the capping layer is a non-ferroelectric material. An electrode is above or below the capping layer. The capping layer is preferably at least 3 nanometers (nm) thick, and preferably resides in direct contact with both the electrode and the ferroelectric material. The capping layer is enriched with a superlattice generator metal, which is usually trivalent bismuth and may also be trivalent thallium. Preferably, the capping layer material is selected from the group consisting of bismuth oxide, bismuth strontate, bismuth tantalate, bismuth niobate, and bismuth niobium tantalate.
Preferably, the capping layer caps the ferroelectric material beneath the top electrode, but if there are two electrodes, such as in a ferroelectric capacitor, the capping layer may cap both the top and bottom of the ferroelectric layer.
In the preferred embodiments, the superlattice generator metal is identical to a superlattice generator metal in the self-ordering layered superlattice material. In preferred embodiments, the ferroelectric layer consists essentially of a bismuth-containing self-ordering layered superlattice material, and the superlattice generator metal consists essentially of bismuth. Particularly preferred layered superlattice materials are selected from the group consisting of strontium bismuth tantalate, strontium bismuth niobate, and strontium bismuth niobium tantalate. The most preferred capping layer material is bismuth oxide.
The capping layer is preferably at least about 3 nm thick, and preferably ranges from 3 nm to 30 nm in thickness, with the most preferred thicknesses ranging from 5 nm to 20 nm to provide adequate defect compensation while being thin enough to avoid significant problems with parasitic capacitance.
The use of the capping layer appears to prevent field screening by providing a transition from the ferroelectric to the conductor. It permits very thin ferroelectric devices with excellent ferroelectric properties. Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.